(1) Field of the Invention
The present invention relates to ac regulators of the type in which an ac input signal is chopped at a frequency which is high compared to the input frequency, typically in excess of 1 kilohertz, and the output of the chopper is then filtered to remove the high frequency component from the signal. By varying the mark to space ratio of the chopper, the output amplitude may be varied.
(2) Description of the Prior Art
AC regulators of the above type have previously been proposed and United Kingdom Pat. No. 1,045,002 discloses a number of arrangements. The basic arrangement shown in this U.K. patent has a chopper connection in series in the load current path. It was found that when the load was other than resistive, distortion and undesired voltage transients could occur at the end of each chopper pulse, and it was proposed in U.K. Pat. No. 1,045,002 to include a shunt path between the chopper and the filter in order to overcome the distortion and transient problem. The shunt path was disclosed as being a further chopper similar to the basic series chopper. The shunt and series chopper were arranged such that when the series chopper was conductive the shunt chopper was non-conductive and vice versa. In one embodiment each of the choppers was composed of a pair of anti-parallel silicon controlled rectifiers, controlled in such a manner that the anti-parallel rectifiers of the shunt chopper were conductive at the same time or the anti-parallel retifiers of the series chopper were conductive at the same time.
It has been found that such a circuit as disclosed in British Pat. No. 1,045,002 does not solve all the distortion and undesired voltage transient problems since it suffers from the problem that if the shunt silicon controlled rectifiers come on before the series silicon controlled rectifiers turn off, large currents can flow from the ac input into the switches. Alternatively if a "dead band" is utilized to prevent the simultaneous conduction and the series rectifiers are turned off before the shunt rectifers are turned on then the inductive energy in the filter has to be controlled by "snubbing" components.
It is an object of the present invention to avoid these problems by providing an electric regulator with an improved switching action.